In telephone systems it is necessary to provide a low impedance dc connection between a pair of wires (the TIP and RING of a customer loop or trunk) for signaling functions, while simultaneously providing a much larger and well controlled ac impedance for voice frequency transmission. Traditionally, this has been accomplished by using a transformer with a low dc resistance and large inductance. One winding of the transformer provides the dc path and the transformer couples the ac signal to the remainder of the circuit. The large inductance (&gt;2H) assures that the transformer does not significantly shunt the ac signal. The disadvantage of this approach is that the transformer must be physically large so that its core can accommodate the net flux created by the dc current without saturating, while still maintaining a large inductance. A small transformer may be used if the dc is blocked from its winding with a capacitor and a shunt dc path is provided. However, the shunt path must also consist of a large dc carrying inductor or an "electronic" inductor. Electronic inductors are smaller but require an operating voltage that degrades the signaling performance of the circuit.
The U.S. Pat. No. 4,243,842, to W. D. Gibb, issued Jan. 6, 1981, proposes a technique for handling the flux problem discussed above. The patent proposes the addition of a balance winding in the transformer, which is connected in series with one portion of the split primary winding. The balance winding is counter-wound with respect to the primary winding and must contain more turns than the n turns of the primary winding. Not only does the added balance winding result in a transformer more expensive than a conventional transformer, but the manufacturing problems in making transformers with part of the primary winding counter-wound with respect to its other part are considerable. Moreover, the patent to Gibb does not deal with the problem of providing a well controlled ac termination impedance to facilitate voice frequency transmission.